1. Field of the Invention
The present invention relates to an image sensing apparatus and imaging system.
2. Description of the Related Art
Image sensing apparatuses each having a pixel array in which a plurality of pixels including photoelectric conversion elements are arrayed in the row and column directions include an X-Y address type CMOS image sensor having an amplifier for each pixel, in addition to a CCD image sensor. The image sensing apparatus of this type is used in an imaging system such as a digital camera or digital video camera. Each pixel in the pixel array is generally covered with a color filter which selectively transmits light of the R (Red), G (Green), or B (Blue) wavelength to guide transmitted light of the wavelength to a pixel below the color filter.
Recently, as the number of pixels of an image sensing apparatus has increased, the pixels of the pixel array have been shrinked. Along with the shrinkage of pixels, the dimension of the aperture region of the pixel comes close to the half-wavelength of light entering the pixel. As a result, it becomes difficult for the pixel to receive light of the R wavelength which is longest among the R, G, and B wavelengths.
To solve this problem, Japanese Patent Laid-Open No. 2007-005629 discloses an image sensing apparatus in which the aperture size of an R pixel is set larger than those of the apertures of pixels of the remaining colors (G and B). According to Japanese Patent Laid-Open No. 2007-005629, this structure can reduce degradation of the image quality caused by shrinkage of pixels.
A case where white balance is adjusted in an imaging system having the image sensing apparatus disclosed in Japanese Patent Laid-Open No. 2007-005629 and a signal processing unit which processes a signal output from the image sensing apparatus to generate image data will be examined. In this case, upon receiving R, G, and B pixel signals output from the image sensing apparatus, the signal processing unit amplifies them with different gains so as to obtain a proper white level in a display or recording image. The gains of the respective colors are often determined to satisfy R gain>G gain>B gain in accordance with the characteristics (spectral characteristic and absorption characteristic) of the color filter, and the wavelength dependence of utilization efficiency of electric charges when, for example, a photoelectric conversion element is arranged on a silicon semiconductor substrate.
Assume that light (stray light) enters at least some pixels of the pixel array at a large incident angle. At this time, the mixture of colors (leakage of signal charges to an adjacent pixel) owing to stray light occurs to the same degree per unit aperture area in two adjacent pixels of different colors (e.g., R and G pixels, or B and G pixels). After output from the image sensing apparatus, pixel signals of different colors containing the mixed color component are amplified with different gains by the signal processing unit. The influence of stray light, i.e., the intensity of the mixed color component generated by stray light differs between colors in an obtained display or recording image.
Particularly in the image sensing apparatus disclosed in Japanese Patent Laid-Open No. 2007-005629, the aperture size of an R pixel is larger than those of pixels of the remaining colors (G and B). Thus, the mixture of colors by stray light in the R pixel becomes more serious than that by stray light in the G and B pixels. When R gain>G gain>B gain, the intensity of a mixed color component in the R pixel signal becomes higher than those of mixed color components in the G and B pixel signals in an image signal amplified by the signal processing unit.
As a result, the influence of stray light on a specific color (R) may stand out in an obtained image.